The present invention relates to servo control methods and apparatus. More particularly, the invention relates to the application of such methods and apparatus in a lithographic projection apparatus.
For the sake of simplicity, the projection system may hereinafter be referred to as the xe2x80x9clensxe2x80x9d; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, catadioptric systems, and charged particle optics, for example. In addition, the first and second object tables may be referred to as the xe2x80x9cmask tablexe2x80x9d and the xe2x80x9csubstrate tablexe2x80x9d, respectively.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the mask (reticle) may contain a circuit pattern corresponding to an individual layer of the IC, and this pattern can then be imaged onto a target area (die) on a substrate (silicon wafer) which has been coated with a layer of photosensitive material (resist). In general, a single wafer will contain a whole network of adjacent dies that are successively irradiated through the reticle, one at a time. In one type of lithographic projection apparatus, each die is irradiated by exposing the entire reticle pattern onto the die in one go; such an apparatus is commonly referred to as a waferstepper. In an alternative apparatusxe2x80x94which is commonly referred to as a step-and-scan apparatusxe2x80x94each die is irradiated by progressively scanning the reticle pattern under the projection beam in a given reference direction (the xe2x80x9cscanningxe2x80x9d direction) while synchronously scanning the wafer table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally less than 1), the speed at which the wafer table is scanned will be a factor M times that at which the reticle table is scanned. More information with regard to lithographic devices as here described can be gleaned from International Patent Applications WO 97/33205 and WO 96/38764, for example.
Up to very recently, apparatus of this type contained a single mask table and a single substrate table. However, machines are now becoming available in which there are at least two independently movable substrate tables; see, for example, the multi-stage apparatus described in International Patent Applications WO 98/28665 and WO 98/40791. The basic operating principle behind such multi-stage apparatus is that, while a first substrate table is underneath the projection system so as to allow exposure of a first substrate located on that table, a second substrate table can run to a loading position, discharge an exposed substrate, pick up a new substrate, perform some initial alignment measurements on the new substrate, and then stand by to transfer this new substrate to the exposure position underneath the projection system as soon as exposure of the first substrate is completed, whence the cycle repeats itself, in this manner, it is possible to achieve a substantially increased machine throughput, which in turn improves the cost of ownership of the machine.
The projection radiation in current lithographic devices is generally UV (ultra-violet) light with a wavelength of 365 nm, 248 nm or 193 nm. However, the continual shrinkage of design rules in the semiconductor industry is leading to an increasing demand for new radiation types. Current candidates for the near future include UV light with wavelengths of 157 nm or 126 nm, as well as extreme UV light (EUV) and particle beams (e.g. electron or ion beams).
In an apparatus as described above, it is necessary to control the relative position of the object tables and the lens to a very high degree of accuracy. Transient inaccuracies in this relative position, which may be caused by vibrations, are therefore particularly problematic. Whilst it may be relatively easy to detect the existence of such vibrations, it requires considerable work to identify and eliminate their sources. Lens vibrations may, for example, be caused by floor vibrations, indirect scanning forces (in the case of step-and-scan devices), noise in vibration isolation systems (originating in pneumatic suspension devices in the apparatus) or acoustic noise, among other things. Since the lens is generally quite large and heavy (e.g. with a mass of the order of about 50-250 kg), it is particularly sensitive to vibrations with a relatively low frequency.
A lithographic projection process may require the positional error of the substrate holder and/or mask holder relative to the lens to be of the order of 2 nm or less. In addition, practical considerations in servo system design can demand that the positional stability of the lens be within tolerances of the order of 1 nm. In tests, the inventors have observed that positional errors of this magnitude can, under certain conditions, be produced by disturbance forces of the order of as little as 1N (acting on a machine that may have a mass of several hundred to several thousand kg). The desired degree of stability can therefore be very difficult to achieve.
It is an object of the present invention to alleviate this problem. More specifically, it is an object of the invention to provide a lithographic projection apparatus in which effective measures are taken to reduce the detrimental effect of lens vibrations on the accuracy with which the substrate and/or reticle tables can be positioned relative to the lens.
According to the present invention, these and other objects are achieved in a lithographic projection which includes
a detection mechanism for detecting accelerations of the projection system, and generating at least one acceleration signal representative thereof, and
a control mechanism responsive to the acceleration signal, for generating at least one control signal to control at least one of the positioning mechanisms so as to move the corresponding object table, thereby to compensate for movements of the projection system.
The present invention also provides a method of controlling the relative position of at least one of the object tables and the projection system in such a lithographic projection apparatus, the method comprising the steps of:
measuring accelerations of the projection system;
determining a force to be applied to at least one of the object tables to cause movement thereof so as to compensate for movements of the projection system;
applying the determined force to that object table.
The feedfoward control provided by the present invention can substantially reduce the effect of vibrations (e.g. in the main frame or base plate of the lithography device) on the relative positions of the lens and object table (wafer table and/or reticle table). This feedforward control can be specifically tuned to provide maximum compensation within particular frequency bands, e.g. around the eigenfrequency of the lens.
The invention is applicable to one or more of the 6 degrees of freedom of the lens, substrate table and/or mask table. For the sake of simplicity, the following discussion will concentrate on a situation whereby correction occurs in only one degree of freedom; however, the presented considerations are equally valid for more degrees of freedom. In this latter case, it will be usual to have a set of detection mechanisms (e.g. one per controlled table per degree of freedom) and to generate several control signals (e.g. one per detection mechanism in the set).
In a preferential embodiment of the invention, the detection mechanisms mounted on the projection system in relatively close proximity (and preferably as close as possible) to the object table/tables whose position is/are to be controlled in response to the control signal. In such a case, a lens acceleration measured by the detection mechanism can be translated with relatively high accuracy into a force to be applied to the object tables(s). On the other hand, the accuracy of the extrapolated required movement of the table(s) is reduced when the detection mechanism is relatively distant from the (controlled) object table(s). In a situation whereby the invention is employed to control the positions of both the reticle table and the wafer table, two (for example) detection mechanisms can be employedxe2x80x94one in proximity to each table.
In the embodiment discussed in the previous paragraph, the detection mechanism comprises a device (such as an accelerometer) which can be affixed to the projection system; such a scenario is discussed below in Embodiment 2, for example. However, in an alternative situation, the detection mechanism comprises an interferometer device which interferometrically measure the relative position and motion of the projection system and at least one of the object tables; such a case is further elucidated below in Embodiment 4, for example.
In a manufacturing process using a lithographic projection apparatus according to the invention, a pattern in a mask is imaged onto a substrate which is at least partially covered by a layer of energy-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book xe2x80x9cMicrochip Fabrication: A Practical Guide to Semiconductor Processingxe2x80x9d, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
Although specific reference may be made in this text to the use of the apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms xe2x80x9creticlexe2x80x9d, xe2x80x9cwaferxe2x80x9d or xe2x80x9cdiexe2x80x9d in this text should be considered as being replaced by the more general terms xe2x80x9cmaskxe2x80x9d, xe2x80x9csubstratexe2x80x9d and xe2x80x9ctarget areaxe2x80x9d, respectively.